According to the prior art, it is known to reproduce audio signals by means of so-called electroacoustic converters. As main components, converters of this kind have a membrane, a drive device, and fastening means for reciprocal connection of the membrane and the drive device. Drive devices for a membrane of this kind are predominantly constituted by a magnet system which has a permanent magnet and a yoke (also called a back iron). The membrane is provided with an oscillator coil and dips into an air gap left in the magnet system. If both ends of the oscillator coil are conductively connected to an audio signal source, the oscillator coil moves in the air gap of the magnet system so that air volumes adjacent to the respective surfaces of the membrane are set into oscillations by way of the membrane connected to the oscillator coil. Since a device of this kind is not suited to satisfactorily reproduce the entire audible frequency band of an audio signal source, development has therefore been switched over to so-called range converters, which are constituted so that they are optimized for only a partial region of the frequency band to be transmitted. As a rule, devices currently in actual use have a converter for low, middle, and high-frequency audio signals. In devices of this kind, the two converters for middle and high-frequency audio signals may also be replace by only one converter. In order to further improve the transmission of low-frequency audio signals, the attempts began quite early to incorporate converters, which were intended for transmitting low-frequency audio signals, into housings in order to prevent obliterations of sound waves which are emitted by the front and back sides of the membrane. In this connection, essentially three basic forms have been produced.
The first basic form, which is also called a closed housing, is distinguished by virtue of the fact that the housing, with the exception of the converter opening in which a converter is mounted, has no other opening. However, the air volume, which is enclosed in the housing and communicates with the back side of the membrane, acts as an additional spring, by means of which the resonance frequency of the converter is increased and consequently the lower cut-off frequency is shifted to higher frequencies. So that sufficiently low sounds can be emitted using devices of this kind, the enclosed air volume, which is also called the damping volume, is not permitted to fall below a minimum value. In devices of this kind, it is disadvantageous that the housing must have at least one wall oriented toward the listening space, which wall, based on its dimensions, is suited to receive the usually large-dimensioned bass converter in the opening. When the space or area requirements of such devices are taken into consideration, these devices cannot be used in applications that also demand a favorable bass reproduction even when there is only a small amount of space available (for example in a motor vehicle).
Bass reflex boxes, which constitute the second basic form, are constructed on the same principle as the first basic form. In contrast to this principle, though, bass reflex boxes have an additional opening in the housing, from which the sound waves emitted backwards from the membrane emerge in-phase with the sound waves emitted from the front side of the membrane and support their action. In bass reflex boxes, the converter and the housing constitute two systems coupled to each other. The bass reflex housing is a resonator whose resonance frequency, which depends on the volume of the housing and the size and depth of the additional opening, is modulated to the natural frequency of the installed bass converter. As a result of this, oscillations of coupled circuits set in. Sounds that are close to the natural resonance of the installed converter excite the gas volume in the housing (disposed in resonance) into particularly powerful sympathetic oscillation. The low coupling frequency that occurs in this connection brings about the fact that bass sounds that can only be weakly transmitted by a converter disposed in a closed housing become audible in an amplified manner.
With bass reflex devices, even if the sound pressure frequency response is not so linear in contrast to closed housing devices, bass reflex devices have the advantage over the closed housing devices that with smaller dimensions, they provide a higher efficiency at low frequencies. However, like closed housing devices, bass reflex devices have the general disadvantage that the converters must be inserted into a wall of the (bass reflex) housing, which rules out a large number of application fields even for these devices (in particular those with a small amount of space available).
For the time being, the so-called band-pass devices represent the end point of box development and at the same time, constitute the third basic form. Band-pass devices are distinguished by virtue of the fact that a closed housing is divided into two chambers by means of a dividing wall. The bass converter is inserted into this dividing wall. In the simplest design, band-pass devices have a bass reflex opening in one of the two chambers and this bass reflex opening is solely responsible for the transmission of sound. The basis for these devices, which demonstrate a very favorable bass reproduction behavior, is the use of the particular frequency response character of a Helmholtz resonator: this device, which is comprised of a hollow chamber with an opening of a particular length and a particular cross section, when excited by means of a converter, demonstrates a frequency response with a distinct resonance frequency at which the sound pressure is maximal and with trailing edges that have an inclination of 12 decibels per octave at lower and higher frequencies. At higher frequencies, however, an additional parasitic effect occurs, which should be compared to the behavior of a transmission line device at the earliest possible time: in the opening of the Helmholtz resonator, standing waves are formed, whose fundamental oscillation has a wavelength of twice the length of the opening. This effect can, however, be prevented through the use of electrical filters.
It is characteristic for the band-pass devices that the volume that is open--because it is provided with the bass reflex opening--requires a chamber volume which, depending on the embodiment, requires between approximately 0.4 and 0.8 of the chamber volume of the volume that is closed--for this consideration. As long as the corresponding volumes are provided, the shape of the different chambers is to a large extent arbitrary. If one also considers the fact that merely for the emergence of sound, band-pass devices require walls that are slightly bigger than the cross section of the bass reflex opening and that otherwise, such devices already permit an acceptable bass reproduction at approximately 10 to 12 liters of total volume, then these devices are also suitable for uses in which there is only a very limited amount of available space. However, if one includes in the consideration the fact that in many intended uses, the limited space available for containing bass reflex devices can also be subjected to a large number of changes, this means that since bass reflex devices react very sensitively to reductions in the total volume, a large number of band-pass devices must called into question due to the space changes.
To explain it in connection with an example: if one assumes that with the so-called standard equipment, the required amount of space for the installation of a bass reproduction device is available under the driver's seat of a passenger vehicle, then, for example when the vehicle is equipped with an electrical seat adjustment, this volume can become no longer available as a result of the required servomotors or can exist only in an altered form. The same is true if the vehicle can be furnished with a spoil seat, for example. When dimensioning a bass reproduction device, even if equipment variants of this kind can be taken into account in the conception of bass reproduction devices, at some point, the time comes when changes (=reductions) in the housing shape or in the bass reproduction device become necessary. In this connection, emphasis need only be placed on the case in which, starting based on a limousine, combination or all-wheel versions are developed (over time), with an altered undercarriage and for the stability of the vehicle, the space under the seats is reduced, for example as a result of reinforcing beads.
Therefore, the object of the invention is to disclose a sound reproduction device for reproducing low-frequency sounds which can very flexibly use an existing available space even when there are changes in this available space and furthermore only requires one sound emission opening to the listening space and the cross sectional area of this opening is smaller than that of the membrane.